Many hardwired communications systems use plug and jack connectors to connect a communications cable to another communications cable or to a piece of equipment such as a computer, printer, server, switch or patch panel. By way of example, high speed communications systems routinely use such plug and jack connectors to connect computers, printers and other devices to local area networks and/or to external networks such as the Internet. FIG. 1 depicts a highly simplified example of such a hardwired high speed communications system that illustrates how plug and jack connectors may be used to interconnect a computer 11 to, for example, a network server 20.
As shown in FIG. 1, the computer 11 is connected by a cable 12 to a communications jack 15 that is mounted in a wall plate 19. The cable 12 is a patch cord that includes a communications plug 13, 14 at each end thereof. Typically, the cable 12 includes eight insulated conductors. As shown in FIG. 1, plug 14 is inserted into an opening or “plug aperture” 16 in the front side of the communications jack 15 so that the contacts 21 or “plug blades” of communications plug 14 mate with respective contacts of the communications jack 15. If the cable 12 includes eight conductors, the communications plug 14 and the communications jack 15 will typically each have eight contacts. The communications jack 15 includes a wire connection assembly 17 at the back end thereof that receives a plurality of conductors (e.g., eight) from a second cable 18 that are individually pressed into slots in the wire connection assembly 17 to establish mechanical and electrical connections between each conductor of the second cable 18 and a respective one of a plurality of conductive paths through the communications jack 15. The other end of the second cable 18 is connected to a network server 20 which may be located, for example, in a telecommunications closet of a commercial office building. Communications plug 13 similarly is inserted into the plug aperture of a second communications jack (not pictured in FIG. 1) that is provided in the back of the computer 11. Thus, the patch cord 12, the cable 18 and the communications jack 15 provide a plurality of electrical paths between the computer 11 and the network server 20. These electrical paths may be used to communicate electrical information signals between the computer 11 and the network server 20.
Industry standards such as, for example, the ANSI/TIA-568-C.2 standard approved Aug. 11, 2009 by the Telecommunications Industry Association (also known as the “Balanced Twisted-Pair Telecommunications Cabling and Components Standards”), have been developed that help ensure that plug and jack connectors manufactured by different vendors will work together and meet minimum performance criteria. Many of these standards specify that the plug and jack connectors conform to the “RJ-45” interface specification. Pursuant to this specification, each plug and jack connector includes eight conductive paths, which are arranged as four differential pairs of conductive paths, and eight interface contacts (which are typically referred to as the “blades” of the plug and the “jackwire contacts” of the jack). Thus, each plug, jack and cable segment in FIG. 1 includes four differential pairs, and thus a total of four differential transmission lines are provided between the computer 11 and the server 20 that may be used to carry two way communications therebetween (e.g., two of the differential pairs may be used to carry signals from the computer 11 to the server 20, while the other two may be used to carry signals from the server 20 to the computer 11). In the connection region where the blades of the plug mate with the jackwire contacts (the “plug-jack mating region”), the eight plug blades 21 (see FIG. 1) and the eight jackwire contacts 1-8 (see FIG. 2) are aligned in a row. FIG. 2 also shows the assignment of the jackwire contacts 1-8 to the four differential pairs. Communications systems that use RJ-45 plugs, jacks and cables are often referred to as “Ethernet” communications systems.
When signals are transmitted over a conductor (e.g., an insulated copper wire) in a communications cable, electrical noise from external sources may be picked up by the conductor, degrading the quality of the signal. In order to counteract such noise sources, the information signals in the above-described communications systems are typically transmitted between devices over communications cables that include one or more pairs of conductors (hereinafter a “differential pair” or simply a “pair”) rather than over a single conductor. The two conductors of each differential pair are twisted tightly together in the communications cables and patch cords so that the eight conductors are arranged as four twisted differential pairs of conductors. The signals transmitted on each conductor of a differential pair may, for example, have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors of the pair. When the signal is transmitted over a twisted differential pair of conductors, each conductor in the differential pair often picks up approximately the same amount of noise from these external sources. Because approximately an equal amount of noise is added to the signals carried by both conductors of the twisted differential pair, the information signal is typically not disturbed, as the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, and this subtraction process may mostly cancel out the noise signal.
Referring again to FIG. 2, it can be seen that the arrangement of the eight jackwire contacts 1-8 in the plug mating region will result in unequal capacitive and/or inductive coupling between the four differential pairs. These unequal couplings between the conductors of different differential pairs give rise to another type of noise that is known as “crosstalk.”
In particular, “crosstalk” refers to unwanted signal energy that is capacitively and/or inductively coupled onto the conductors of a first “victim” differential pair from a signal that is transmitted over a second “disturbing” differential pair. The induced crosstalk may include both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both types of crosstalk comprise an undesirable noise signal that interferes with the information signal that is transmitted over the victim differential pair.
In order to reduce the effects of crosstalk, communications jacks now routinely include compensating crosstalk circuits that introduce compensating crosstalk that is used to cancel much of the “offending” crosstalk that is introduced in the plug and in the plug-jack mating region as a result of the industry-standardized connector configurations and interoperability standards. These crosstalk compensation circuits are typically implemented by routing conductors of two different differential pairs in a jack close to each other to intentionally create capacitive and/or inductive coupling that cancels capacitive and inductive couplings that arise between the conductors of the two differential pairs in other portions of the mated plug-jack connection.
Another important parameter for the above-described plug and jack connectors is the return loss that is experienced along each differential transmission line through the connector. The return loss of a transmission line is a measure of how well the transmission line is impedance matched to the system reference impedance (typically 100 ohms for balanced twisted-pair cabling systems). Deviations from this reference impedance by the transmission line or a terminating device or other loads that are inserted along it may result in impedance discontinuities that will cause undesirable signal reflections. In particular, the return loss is a measure of the signal power that is lost due to such signal reflections that may occur at discontinuities (impedance mismatches) in the transmission line. Return loss is typically expressed as a ratio in decibels (dB) as follows:
      R    ⁢                  ⁢          L      ⁡              (        dB        )              =      10    ⁢                  ⁢          log      10        ⁢                  P        i                    P        r            where RL(dB) is the return loss in dB, Pi is the incident power and Pr is the reflected power. High return loss values indicate a good impedance match (i.e., little signal loss due to reflection), which results in lower insertion loss values, which is desirable.
In Ethernet communications systems, each transmission line in the cables and connectors, and the terminating devices, will typically be designed to have a differential impedance of 100 ohms, if possible, in order to obtain high return loss values. The industry standards typically specify minimum return loss requirements for the transmission lines within individual connectors, within mated connectors (i.e., across a mated plug and jack) and/or for an entire communications channel (i.e., for one or more differential transmission lines that extend from computer 11 to server 20 in FIG. 1 across various connectors and cable segments). As return loss typically decreases with increasing frequency (i.e., the return loss performance gets worse with increasing frequency), the industry standards typically specify minimum return loss values that must be met as a function of frequency for the specified components and/or channels.